1. Field of the Invention
The present invention relates in general to an image forming apparatus which forms a developer image on an image bearing body by developing, with a charged developer, an electrostatic latent image formed on the image bearing body into a visible image, and transfers the developer image to a recording medium.
2. Discussion of Related Art
As an image forming apparatus, an electrophotographic printer is conventionally known. In this type of image forming apparatus, a photosensitive body as an image bearing or carrying body is exposed to a light to form an electrostatic latent image on the photosensitive body, and a charged developer or developing agent such as toner is applied to the photosensitive body to thereby develop the electrostatic latent image into a visible image, so that a developer image as the visible image is formed on the photosensitive body.
In applying the developer to the photosensitive body, a developing bias is supplied to a developer bearing or carrying body (such as a developing roller) of a developing device which is held in abutting contact with the photosensitive body, so as to form, between the developer bearing body and the photosensitive body, an electric field in a direction in which the developer moves toward the photosensitive body, whereby the developer is applied to the photosensitive body. Thereafter, a recording paper as a recording medium is fed between the photosensitive body and a transfer body such as a transfer roller which is disposed so as to be opposed to the photosensitive body, and a transfer bias is supplied to the transfer body, so as to form, between the transfer body and the photosensitive body, an electric field in a direction in which the developer moves toward the recording paper, so that the developer image on the photosensitive body is transferred to the recording paper.
In the thus arranged image forming apparatus, it is general that the charge amount of the developer, in detail, for instance, charge-to-mass ratio of the developer represented by the charge amount of the developer per unit mass gradually changes as the printing operation is repeated. When the charge amount changes, the optimum value of the transfer bias to be supplied to the transfer body for the transfer of the developer image from the photosensitive body to the recording paper changes. In view of this, the transfer bias value (current value or voltage value) to be supplied to the transfer body is controlled depending upon a number of printed sheets of paper, as disclosed in JP-A-5-158357, for instance.